1. Field of the Invention
The present invention relates to an optical fiber type wavelength converter using Cherenkov radiation type of phase matching.
2. Description of the Prior Art
There is known a light wavelength converter in the form of an optical fiber (hereinafter referred to as "fiber") structured of a nonlinear crystalline core covered with a clad and using Cherenkov radiation type of phase matching. Such light wavelength converter is also known as an optical fiber type second harmonics generator (hereinafter referred to as "SHG"). Since, in the Cherenkov radiation type, second harmonic wave (hereinafter referred to as "SH wave") can be generated with phase matching of light performed virtually automatically, SHG is used as a short wavelength light source and the like.
As shown in FIG. 1, a short wavelength light source comprises a semiconductor laser 1, a coupling lens 2 for converging beam of light radiated from the semiconductor laser thereby introducing the light into an SHG through its side face, and SHG 3 whose core is made of a nonlinear crystal, and an axicon 4 for adjusting the wave front of an SH wave converted by and radiated from the wavelength converter thereby turning the SH wave into a parallel beam of light.
FIG. 2A and FIG. 2B are conceptual diagrams of such SHG. The SHG is composed of a cylindrical core 10 and a cylindrical clad layer 20 concentrically surrounding the core 10.
Referring to FIG. 2A, when a fundamental mode propagates through the core 10 having an effective refractive index N(.omega.) from left to right in the diagram, a nonlinear polarization wave generating an SH wave also propagates at the same phase velocity C/N(.omega.) (C: light velocity). Let it be supposed now that the nonlinear polarization wave generates an SH wave at the point A in the diagram in the direction at an angle .theta. with the waveguide direction and, after a unit time, it generates an SH wave again at the point B in the direction .theta. the same as before. If, for example, the SH wave generated at the point A propagates through the clad layer 20 and reaches the point C after the unit time has passed and if the angle .theta. is such an angle to allow AC to cross BC at a right angle, then, the wave front of the SH wave generated by the nonlinear polarization wave at the interval AB becomes BC, which means that a coherent SH wave has been generated.
The SH wave generated as described above propagates as a clad mode repeating total reflection at the boundary surface between the clad layer 20 and the air as shown in FIG. 2B and is emitted from the end face of the fiber in the form of a cone determined by the angle .theta.. The wave front of the thus emitted wave constitutes a form of cone with the center axis of the fiber as its axis.
Such an SHG as shown in FIGS. 2A and 2B that has a length allowing the generated SH wave to be reflected by the boundary surface between the clad and the air and returned to the core again has hitherto been considered good. This is because it has been considered that an SH wave returned to a point interferes with an SH wave generated at the point and, as a result, they are extinguished. Since the conversion efficiency has been considered proportional to the fiber length, there has been used a fiber with a larger clad diameter for obtaining enhanced conversion efficiency.
Further, since it is considered that an enhancement of conversion efficiency can be obtained if the nonlinearity constant of the nonlinear material filled in the core is large, there has been an attempt, as a means to improve the wavelength conversion efficiency of such SHG, to select an optimum core diameter and select an optimum material for the clad glass.
However, fundamentally, the conversion efficiency largely depends on the degree of refractive dispersion of materials and, hence, there has been a limit to the enhancement of the efficiency attainable by the conventional means.